The present invention relates to registration control in various production methods that use imaging. This includes, but is not limited to, Electronics Components (ELC), Flat Panel Displays (FPD), various conductive and non-conductive patterns or holes on/in PCBs, masks for soldering, plating, etching or plasma etching, or hole plug curring. The invention is particularly useful in producing multi-layer PCBs in accordance with the laser direct imaging (LDI) process, and is therefore described below particularly with respect to this process.
Multi-layer PCBs (sometimes referred to as PWBs or printed wiring boards) are widely used in electronic systems. Such PCBs may include as many as 20 or 30 layers, each having its own electrically conductive pattern, with various layers electrically connected together by interlayer connections, such as pins, posts, plated-through-holes (PTHs), pads and the like. The interlayer registration of the conductive patterns is of critical importance and becomes increasingly problematical as the dimensions of the conductive patterns decrease in size, and as the panels increase in size to improve productivity.
In the conventional phototool process for producing PCBs, each layer is imaged via a phototool film on a photoresist layer. As the dimensional tolerances of the conductive patterns become tighter, not only must the phototools be made to the tighter tolerances, but these tolerances must also be maintained throughout the lifetime of the phototool. However, phototools of the plastic type tend to change in dimensions with changes in temperature and/or humidity, and phototools of the glass plate type are expensive and hard to handle.
As a result, imagining processes based on laser direct imaging (LDI) have become increasingly used. In LDI processes, the conductive pattern is imaged directly on the photoresist by lasers. Each conductive pattern is defined by image data, e.g., from a CAM file, to be applied on the photoresist according to scanning control data which controls the relative movements between the laser exposure head and the substrate. However, the positional and dimensional accuracy, and the repeatability of image placement on the photoresist, become critical.
It is therefore common to use fiducials, (i.e., references) for precise registration of the patterns in the various layers. To provide positional accuracy in the various layers, the deviations between the actual locations and the nominal locations of such fiducials are determined and used to calculate position and scaling factors, which are in turn used for correcting the locations of the image data. Examples of known techniques for providing accuracy are described in U.S. Pat. Nos. 5,894,350; 5,506,793; 5,548,372; 5,403,684; 5,381,004; 5,200,800; 5,682,243 and 5,980,088, and the publication titled xe2x80x9cLaser Direct Imaging for Precise Positioning of Circuitry with HDI/MicroVias and SBU Technologies, by Guenther Enne, et al., TPCS 2000 Forum Proceedings, Nov. 23-25, 2000, pp 377-384. However, if such correction factors are produced and used for correcting the locations of each image data in each layer, the required database and processing time would both be extremely large.
A broad object of the present invention is to provide another method of registration control between the layers in a multi-layer PCB. Another object is to provide a method of imaging an overlying conductive pattern over an underlying conductive pattern in order to reduce misregistration; and a further object of the invention is to provide an improved method of making a PCB by LDI (laser direct imaging) enabling a high degree of registration control to be attained.
According to one aspect of the present invention, there is provided a method of imaging an overlying pattern over an underlying pattern on a substrate, each pattern being defined by image data to be applied at nominal (i.e., theoretical or desired) locations according to scanning control data, the method comprising: determining deviations between the actual locations, and the nominal locations of predetermined reference targets of the underlying conductive pattern on the substrate; and utilizing the determined deviations for modifying the scanning control data used for imaging the image data of the overlying conductive pattern in order to reduce misregistration thereof with respect to the underlying conductive pattern.
As will be described more particularly below, such a method enables each conductor pattern layer to be registered with respect to its preceding conductive layer, without requiring an inordinate database or processing time, since the determined deviations are utilized to modify the scanning control data, rather than the image data file.
According to further features in the described preferred embodiment, the reference targets are predetermined features, preferably predetermined connection sites, in the underlying conductive pattern. Thus, instead of using two, three or four fiducials as the reference targets for registration purposes, as characteristic of the prior art, the novel method enables a much larger number of reference targets to be used, thereby enhancing the overall registration process. Also, since the electrical connections are made through the layers at the connection sites, selecting connection sites as the reference targets for registration purposes better assures accurate registration at the critical locations where the interlayer electrical connections are to be made, and increases the level of misregistration tolerated by the process.
The underlying conductive pattern used for determining the deviations between the actual and the nominal (i.e., the theoretical or desired) locations of the predetermined reference targets would usually be the one in the immediately underlying layer, but could also be one in a more distant underlying layer particularly if electrical connections are to be made from that layer to the overlying conductive pattern to be applied.
According to further features in the described preferred embodiment, the predetermined features selected as reference targets may be assigned different weights according to their registration tolerances, and the deviations may be determined according to a threshold which varies with the weight assigned to the respective reference target. This feature also increases the level of misregistration tolerated by the process.
According to still further features in the described preferred embodiment, each of the conductive patterns includes a global area having predetermined reference targets, and one or more local areas each having predetermined reference targets; the reference targets in the global area of the underlying conductive pattern being used for imaging the local area of the overlying conductive pattern with respect to the underlying conductive pattern; the determined deviations in the reference targets in the local area of the underlying conductive pattern being used for modifying the scanning control data to reduce misregistration of the imaged local area of the overlying conductive pattern with respect to the underlying conductive pattern.
According to another aspect of the present invention, there is provided a method of making a PCB having a plurality of layers containing conductive patterns overlying each other and to be precisely located with respect to each other, the method comprising: applying each layer by an LDI process in which each conductive pattern is defined by image data to be applied at nominal locations according to scanning control data; determining deviations between the actual locations and the nominal locations of predetermined reference targets of each conductive pattern on a substrate before the overlying conductive pattern is applied thereover; and utilizing the determined deviations for modifying the scanning control data used for imaging the image data of each overlying conductive pattern in order to reduce misregistration thereof with respect to its respective underlying conductive pattern.
According to a still further aspect of the invention, there is provided apparatus for imagining conductive patterns on a substrate, comprising: a holder for holding the substrate; an imaging device for imaging at least two conductive patterns in overlying relationship to each other on the substrate, each conductive pattern being defined by image data to be applied at nominal locations on the substrate according to scanning control data; scanning means for effecting relative movement between the substrate and the imaging device according to the scanning control data for the respective conductive pattern; a sensor for sensing the actual locations of predetermined reference targets of an underlying conductive pattern on the substrate before an overlying conductive pattern is applied thereover; and a controller programmed to determine deviations between the actual locations and the nominal locations of predetermined reference targets of an underlying conductive pattern on the substrate and to utilize the determined deviations for modifying the scanning control data used for imaging the image data of the overlying conductive pattern in order to reduce misregistration thereof with respect to the underlying conductive pattern.
Further features and advantages of the invention will be apparent from the description below.